This invention relates to the removal of naphthenic acids from hydrocarbon feedstocks. More particularly, it relates to the removal of naphthenic acids from diverse liquid hydrocarbon feedstocks, especially as part of a sweetening process for the feedstock. Because of the rather particularized nature of our invention, it appears desirable to expound on certain current process characteristics so that the contributions of the present invention in advancing the relevant an can be better appreciated.
Many hydrocarbon streams have sulfur-containing compounds as undesirable components whose removal constitutes an important stage of hydrocarbon processing. Where these components are mercaptans their "removal" is generally only a conversion of mercaptans to disulfides which remain in the feedstock as inoffensive components of the hydrocarbon stream, a process usually referred to as "sweetening" (with the initial mercaptan-laden stream referred to as "sour" feedstock). The conversion of mercaptans to disulfides often is accomplished merely through air oxidation as catalyzed by various metal chelates; see J. R. Salazar in "Handbook of Petroleum Refining Processes", R. A. Meyers, editor, pages 9-3 to 9-13. But catalysis of mercaptan oxidation proceeds best in an alkaline environment--and therein hangs our tale.
The prior an has required a highly alkaline environment, typically achieved by strong bases such as alkali metal hydroxides (for example, caustic soda). Unfortunately, the caustic does not merely provide an alkaline environment but in time is neutralized by acidic components of the hydrocarbon stream, requiring its continued replacement and replenishment. Disposal of spent caustic solutions is itself an environmental problem, and proper disposal may exact a heavy financial penalty on the sweetening process. This is especially true for certain feedstocks, such as kerosene, which typically have a significant content of naphthenic acids.
Naphthenic acids are carboxylic acids found in petroleum and various petroleum fractions during their refining; see Kirk Othmer, "Encyclopedia of Science and Technology", 3rd Edition (1981), pp 749-53. Naphthenic acids are predominantly monocarboxylic acids having one or more cycloaliphatic groups alkylated in various positions with short chain aliphatic groups and containing a polyalkylene chain terminating in the carboxylic acid function. Although cyclopentane rings are the predominant cycloaliphatic ring structure, other cycloaliphatics tings, such as cyclohexanes, also may be present in appreciable quantities. The predominant acids are represented in Kirk Othmer by the formula, ##STR1## where n may range from 1 to 5, m is greater than 1, and R is a small aliphatic group, predominantly a methyl group. Since naphthenic acids are well known in the art their further characterization here is unnecessary and the interested reader may consult appropriate texts for additional information.
The naphthenic acid content of feedstocks such as kerosene engenders further complications arising from the limited solubility of alkali metal naphthenates in concentrated alkali. One consequence is that when a caustic--wet fixed bed oxidation catalyst is used--a common and otherwise economically favored variant--formation of insoluble alkali metal naphthenates tends to cause bed plugging. To avoid this, kerosene and kerosene-like feedstocks undergo a caustic prewash to remove naphthenic acids prior to entry of the feedstock to the fixed bed. But the solubility characteristics of the alkali metal naphthenates are such that their efficient extraction from kerosene-type feedstocks into aqueous media requires utilization of a dilute caustic (usually under 3 weight percent) prewash, which increases the volume of the spent caustic and further intensifies its disposal problem.
Although naphthenic acids are troublesome in the sweetening process they do have significant value as precursors to wood preservatives, oil-based paint dryers, surfactants, corrosion inhibitors, and lubricant additives. Their recovery is highly desirable, but in the scenario described above they must be recovered from a dilute aqueous solution, which imposes yet another financial burden.
The dilemma faced by a processor with the need to sweeten the liquid hydrocarbon feedstocks, and especially kerosene-type feedstocks, is multifaceted. The most desirable sweetening process which converts mercaptans to disulfides operates best in a caustic environment. The naphthenic acids in feedstocks previously have been removed in a caustic prewash to avoid reactor bed plugging, but the limited solubility of alkali metal naphthenates requires the use of dilute alkali, which exacerbates the disposal problem of spent caustic solutions. Although the naphthenic acids themselves are valuable commodities whose recovery might otherwise offset spent caustic disposal costs their recovery from dilute alkali is difficult and expensive, with little if any economic return. The result is that a high naphthenic acids content in a hydrocarbon feed imposes economic burdens on an otherwise simple chemical process.
The keystone of our invention is the recognition that certain metal oxide solid solutions related to hydrotalcite are effective adsorbents for naphthenic acids. This property permits the efficient removal of naphthenic acids from kerosene-type feedstocks specifically, and hydrocarbon feedstocks generally, using an adsorbent bed of the metal oxide solid solution prior to the sweetening process itself. Where adsorption of naphthenic acids is coupled with a process for desorption to regenerate the metal oxide solid solution it may be possible to recover the naphthenic acids themselves in a suitably concentrated form well adapted for a commercially economical naphthenic acid recovery program.
Before proceeding it appears advisable to avoid semantic confusion by defining several terms. The anionic clay known as hydrotalcite is a layered double hydroxide of ideal composition Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O where the carbonate anion is intercalated between infinite brucite-like sheets. Although "hydrotalcite" is most properly applied to a clay of composition which is Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O often it has been used to describe related layered double hydroxides with varying Mg/Al ratios. However, at least when the number ratio of Mg/Al atoms is less than 3, after calcination such materials are better described as solid solutions of magnesium oxide and aluminum oxide. That is, calcination destroys the layered structure characteristic of hydrotalcite and affords a solid solution. But the terminology as applied to such solid solutions often retains the "hydrotalcite" name, as in, for example, "synthetic hydrotalcites". In this application henceforth w shall try to be consistent in using the term "metal oxide solid solution" (occasionally referred to by the acronym MOSS) to describe such calcined synthetic materials. The second point involves the use of the term "Mg/Al" and analogous terms. In this application Mg/Al shall be the number ratio of magnesium to aluminum atoms in a solid solution of magnesium oxide and aluminum oxide. Others have used a different definition for the Mg/Al ratio.
Hydrotalcites, and more usually "calcined hydrotalcites," i.e., the metal oxide solid solutions formed in the calcination of hydrotalcites, have been used as adsorbents of anions, especially anions of complexed metals, but only in aqueous solution. For example, the patentee in U.S. Pat. No. 5,055,199 used as an adsorbent a "calcined hydrotalcite" of general formula A.sub.6 B.sub.2 (OH).sub.16.4H.sub.2 O, where A is a divalent cation of magnesium, nickel, iron, or zinc, B is a trivalent cation of aluminum, iron, or chromium, and C is a cation such as hydroxide, carbonate, nitrate, and halide. The hydrotalcite calcined at 400.degree.-650.degree. C. was effective in lowering amounts of cyanide, thiocyanate, thiosulfate, citrate, or EDTA complexes of various metals from aqueous streams; cf. U.S. Pat. Nos. 4,744,825, 4,752,397, 4,935,146, and 5,068,095, all of a common assignee, for related teachings. The critical observation is that all of these teachings refer to adsorption from aqueous solutions; to the best of our knowledge there is no art relating to adsorption by "calcined hydrotalcites" of materials from non-aqueous streams, particularly hydrocarbon streams.